Suppression of galactosylceramidase (GALC) expression in the twitcher mouse model of globoid cell leukodystrophy (GLD) is caused by nonsense-mediated mRNA decay (NMD)

Abstract

The twitcher mouse is a pathologically and enzymatically authentic model of globoid cell leukodystrophy (GLD, Krabbe disease) that has been widely used for the evaluation of potential therapeutic approaches. This naturally occurring mouse model contains a premature stop codon (W339X) in the galactosylceramidase (GALC) gene that abolishes enzymatic activity. Using either immunocytochemical approaches or Western blot methodology, we have been unable to detect the truncated form of GALC expected to be produced in these animals. Nonsense-mediated mRNA decay (NMD) is a cellular protection mechanism that degrades newly synthesized transcripts containing a premature termination codon (PTC). Since the naturally occurring mutation in the twitcher mouse introduces a PTC, we hypothesized that NMD might affect the degradation of GALC mRNA in these animals. Consistent with this hypothesis, we determined that the amount of GALC transcript was inversely proportional to the number of twitcher containing alleles. Similar reductions in GALC mRNA were detected in a twitcher-derived Schwann cell line (TwS1) when compared to wild-type Schwann cells (IMS32). Anisomycin, emetine and puromycin, inhibitors of NMD, effectively increased the level of GALC transcript in the TwS1 cells providing further support for nonsense-mediated mRNA decay being the mechanism by which no GALC protein is detected in these animals. Understanding the mechanistic differences between the lack of enzymatic activity in the twitcher model and that observed with the missense mutations that cause human disease yields not only novel therapeutic insights but also highlights the need for additional animal models.